Current sources and current sinks are commonly used to provide regulated currents in circuits of all types. As shown in FIG. 1A, a current sink can be constructed as a combination of a sense resistor, a MOSFET and an operational amplifier. The operational amplifier adjusts the voltage at the gate of the MOSFET to minimize the voltage difference between the inputs of the op amp. In a perfect system, the voltage at the source of the MOSFET, Vs, equals the voltage on the positive terminal of the amplifier, Vset, and the current is given by I=Vset/R. FIG. 1B shows a current source constructed using a similar combination of components.
For some applications, it is desirable to use a series of current sinks or sources driven using the same set voltage, Vset. In an arrangement of this type, each current sink or current source defines a separate channel for current flowing to ground. For the currents in each channel to be equal, all duplicated elements must exactly match in value and characteristics. Unfortunately, mismatches inevitably result because manufacturing variations are unavoidable. Though mismatch between sense-resistors can be minimized with careful layout, random offset within each amplifier is more difficult to correct and can contribute directly to mismatch between channel currents. In fact, random offset is often the main contributor to mismatch—particularly where R is small since I=Vset/R+VOS/R. Consider for example, a hypothetical low power implementation where R is 2 Ohms. If Vos is in the range of −10 mV to 10 mV, then VOS/R can be as large as 5 mA. This would be significant for the case where Vset/R is 20 mA (which would not be unusual for low power devices).
For this reason, U.S. patent application Ser. No. 10/970,061 (incorporated in this document by reference) describes a method for sharing a single operational amplifier between a series of channels. As shown in FIG. 2, this method uses two multiplexers. The first allows the output of an operational amplifier to be switched between channels. The second multiplexer allows the feedback voltage to the operational amplifier to be switch in the same fashion. The overall result is that the operational amplifier is shared, with each channel being selected in a (typically) rotating sequence. A problem encountered with this method arises because the operational amplifier takes time to adapt as it is switched between channels. If two channels are operating at significantly different values, regulation of the channel selected second will be bobbled as the operational amplifiers adapts to conditions of the second channel. The second channel starts with the conditions from the previous channel and then the current has to be changed to the final desired value.